Method for adjusting resonance frequencies of a vibrating microelectromechanical device

ABSTRACT

The present invention relates to a method for adjusting the resonant frequencies of a vibrating microelectromechanical (MEMS) device. In one embodiment, the present invention is a method for adjusting the resonant frequencies of a vibrating mass including the steps of patterning a surface of a device layer of the vibrating mass with a mask, etching the vibrating mass to define a structure of the vibrating mass, determining a first set of resonant frequencies of the vibrating mass, determining a mass removal amount of the vibrating mass and a mass removal location of the vibrating mass to obtain a second set of resonant frequencies of the vibrating mass, removing the mask at the mass removal location, and etching the vibrating mass to remove the mass removal amount of the vibrating mass at the mass removal location of the vibrating mass.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/255,576, filed on Oct. 21, 2008, now U.S. Pat. No.8,327,684, the entire contents of which is incorporated herein byreference.

BACKGROUND

1. Field

The present invention relates to a method for adjusting the resonantfrequencies of a vibrating microelectromechanical device.

2. Related Art

Vibrating masses are commonly used elements in microelectromechanical(MEMS) devices such as MEMS resonators and resonant inertial sensors.These microfabricated resonators can be used in MEMS gyroscopes to sensethe rotation of the device by measuring changes in vibrationalamplitudes upon rotation. In typical vibrating mass gyroscopes, thedevice may be driven in one axis and the vibrational amplitude sensed inanother axis. An example of a resonator structure used in a MEMSgyroscope is the Disc Resonator Gyroscope (DRG) described in U.S. Pat.No. 7,347,095 entitled “Integral Resonator Gyroscope” and U.S. PatentApplication Pub. No. 2007/10017287. The resonant frequencies of thedevice in these two axes are typically required to be identical foroperation, and are designed to have common frequencies. However, theprocess for manufacturing MEMS resonators typically produces deviceswith resonant frequencies which are not precisely at the desiredresonant frequency value for each vibratory axis due to productiontolerances. These differences between the resonant frequencies of theMEMS resonator in the drive and sense axes are commonly called frequencysplits. These splits are typically tuned into coincidence by anelectronic or electromechanical means to enable device operation.Correction methods can be performed to adjust the resonant frequenciesof a MEMS resonator in order to correct for frequency splits. However,such correction methods may over or under correct the resonantfrequencies and thus do not produce the level of precision necessary toadequately adjust the resonant frequencies of the MEMS resonator. If thefrequency split of the MEMS resonator is too large, that is, theresonant frequencies of the MEMS resonator in its operational axesdeviate too much from the desired resonant frequencies coincident value,then the MEMS resonator may be inaccurate or be unsuitable for itspurpose. Further, the method for implementing these corrections may beincompatible with repeatable volume manufacturing processes.

Thus, there is a need for a method to more efficiently and accuratelyadjust the resonant frequencies of a vibrating microelectromechanicaldevice to reduce the frequency split of the resonator device.

SUMMARY

In one embodiment, the present invention is a method for adjusting theresonant frequencies of a vibrating mass including the steps ofpatterning a surface of a device layer of the vibrating mass with amask, etching the vibrating mass to define a structure of the vibratingmass, determining a first set of resonant frequencies of the vibratingmass, determining a mass removal amount of the vibrating mass and a massremoval location of the vibrating mass to obtain a second set ofresonant frequencies of the vibrating mass, removing the mask at themass removal location, and etching the vibrating mass to remove the massremoval amount of the vibrating mass at the mass removal location of thevibrating mass.

In another embodiment, the present invention is a method for adjustingthe resonant frequencies of a vibrating MEMS device including the stepsof patterning a surface of the vibrating MEMS device with photoresist,the photoresist having open areas located where the MEMS device shouldbe etched, etching the vibrating MEMS device at locations correspondingto the open areas, determining a first resonant frequency of thevibrating MEMS device along a first axis and a second resonant frequencyof the vibrating MEMS device along a second axis, determining a firstmass removal amount of the vibrating MEMS device and a first massremoval location of the vibrating MEMS device to alter the firstresonant frequency and reduce a resonant frequency difference betweenthe first resonant frequency and the second resonant frequency, removingthe photoresist at the first mass removal locations using laserablation, and etching the vibrating MEMS device to remove the first massremoval amount of the vibrating mass at the first mass removal locationof the vibrating MEMS device.

In yet another embodiment, the present invention is a method foradjusting resonant frequencies of a vibrating MEMS device including thesteps of determining a first resonant frequency of the vibrating MEMSdevice along the first axis and a second resonant frequency of thevibrating MEMS device along the second axis, determining a mass removalamount of the vibrating MEMS device and a mass removal location of thevibrating MEMS device to alter the first resonant frequency and reduce aresonant frequency difference between the first resonant frequency andthe second resonant frequency, coating a surface of the vibrating MEMSdevice with a conformal masking material, removing the masking materialat the mass removal location using laser ablation, and etching thevibrating MEMS device using deep reactive ion etching to remove the massremoval amount of the vibrating MEMS device at the mass removal locationof the vibrating MEMS device.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings, wherein:

FIG. 1 is a flow chart of an embodiment of the present invention;

FIG. 2 is a side view of a vibrating mass;

FIG. 3 is a side view of a vibrating mass;

FIG. 4 is a side view of a vibrating mass;

FIG. 5 is a side view of a vibrating mass;

FIG. 6 is a top view of a vibrating mass;

FIG. 7 is a flow chart of an alternate embodiment of the presentinvention;

FIG. 8 is a side view of a vibrating mass;

FIG. 9 is a side view of a vibrating mass;

FIG. 10 is a side view of a vibrating mass;

FIG. 11 is a side view of a vibrating mass; and

FIG. 12 is a side view of a vibrating mass.

DETAILED DESCRIPTION

Methods and systems that implement the embodiments of the variousfeatures of the present invention will now be described with referenceto the drawings. The drawings and the associated descriptions areprovided to illustrate embodiments of the present invention and not tolimit the scope of the present invention. Reference in the specificationto “one embodiment” or “an embodiment” is intended to indicate that aparticular feature, structure, or characteristic described in connectionwith the embodiment is included in at least an embodiment of the presentinvention. The appearances of the phrase “in one embodiment” or “anembodiment” in various places in the specification are not necessarilyall referring to the same embodiment. Throughout the drawings, referencenumbers are re-used to indicate correspondence between referencedelements. In addition, the first digit of each reference numberindicates the figure in which the element first appears.

FIG. 1 is a flow chart of an embodiment of the present invention. InStep S102, the process to adjust the resonant frequencies of a vibratingmass begins. As shown in FIG. 2, a surface 4 of a device layer 22 ofvibrating mass 2 is patterned with a mask 6, in Step S104. Mask 6 canbe, for example, an etch mask such as photoresist. Device layer 22 canbe connected to a supporting substrate 12 that may provide mechanicaland electrical interconnection to the movable elements of vibrating mass2 as well as to the stationary elements that serve as, for example,drive and sense electrodes. Open areas 8 can be formed in mask 6corresponding to locations in device layer 22 where material should beremoved in device layer 22 to define a device structure of vibratingmass 2. Open areas 8 in the mask 6 can be formed by photolithographicprocessing.

Vibrating mass 2 can be any mass that vibrates and which has resonantfrequencies that need to be adjusted. In one embodiment, vibrating mass2 is a vibrating MEMS device such as a MEMS resonator. In anotherembodiment, vibrating mass 2 is a silicon MEMS resonator. In yet anotherembodiment, vibrating mass 2 is the vibrating element of a MEMS sensor.In still another embodiment, vibrating mass 2 is quartz. Likewise,device layer 22 can be formed from silicon in one embodiment and quartzin another embodiment. It is contemplated that vibrating mass 2 can beused in a gyroscope or any other device where vibrations are required.In one embodiment, vibrating mass 2 is used for navigation such as withvehicles, munitions, or personnel. In another embodiment, vibrating mass2 is used for orientation sensing. Furthermore, vibrating mass 2 can beused undersea or in head tracker systems. Vibrating mass 2 can have anythickness, but in an exemplary embodiment, vibrating mass 2 has athickness between approximately 100 μm to 600 μm.

In one embodiment mask 6 is photoresist. For example, mask 6 can bepositive photoresist, negative photoresist, SU-8 photoresist,photoresist including a mixture of diazonaphthoquinone (DNQ) and novolacresin, deep ultraviolet photoresist, or any other type of resist. Mask 6can be, for example between approximately 2 μm thick to 30 μm thickdepending on the desired etch depth, etch rate, and selectivity tophotoresist etching. In another embodiment, the mask may be an inorganicthin film, such as nickel, patterned by techniques such as etching orliftoff.

In Step S106, vibrating mass 2 is etched at device layer 22 to formtrenches 10 as shown in FIG. 3. In one embodiment, vibrating mass 2 canbe etched using deep reactive ion etching. In another embodiment,vibrating mass 2 is etched using deep reactive ion etching using atime-sequenced etch and passivate chemistry, such as using sulfurhexafluoride, SF6, for the etching and octafluorocyclobutane, C4F8, forthe passivation in a process commonly known as the Bosch process.

In Step S108, vibrating mass 2 is analyzed to determine if the resonantfrequencies of vibrating mass 2 need to be adjusted and locations ofvibrating mass 2 where mass should be removed from device layer 22 toachieve the desired resonant frequencies of vibrating mass 2. Forexample, if vibrating mass 2 has a resonant frequency of approximately14.950 kHz in a first axis and a resonant frequency of approximately14.900 kHz in a second axis, then vibrating mass 2 should be adjusted todecrease the resonant frequency of the first axis by 50 Hz to bring thevalues for both the first axis and the second axis into conformity at acommon resonant frequency of approximately 14.900 kHz.

Adjusting the resonant frequencies of vibrating mass 2 can be done, forexample, by removing select amounts of mass from select locations ofvibrating mass 2. In Step S110, the mass removal amount of device layer22 is determined while in Step S112, the mass removal location of devicelayer 22 is determined. In one embodiment, mask 6 remains on vibratingmass 2 when analysis of vibrating mass 2 is performed. It iscontemplated that since mask 6 may be approximately 6 μm or less thickwhile vibrating mass 2 may be 100 μm to 600 μm thick, thatdisproportional distributions of mask 6 may have a negligible effect onthe analysis of vibrating mass 2. This may be especially true where itis unlikely that mask 6 will be distributed unevenly in a significantmanner throughout vibrating mass 2.

In Step S114, select locations of mask 6 are removed corresponding tothe select locations of vibrating mass 2 as shown in FIGS. 4 and 6. FIG.4 is a side view of vibrating mass 2 while FIG. 6 is a top view ofvibrating mass 2. In FIGS. 4 and 6, trenches 10 expose supportingsubstrate 12. As shown in FIGS. 4 and 6, select holes 14 are created inmask 6 corresponding to select locations where mass from device layer 22should be removed so that vibrating mass 2 can have the desired resonantfrequencies. In one embodiment, mask 6 is removed through laserablation. It is contemplated that by using a low energy laser ablation,the amount of mask 6 that is removed can be better controlled whencompared with a high energy laser ablation. Furthermore, with the use ofa low energy laser ablation, it is contemplated that less debris can becreated. With less debris, there is less chance that the debris willaffect the operation and yield of the device. At low energies, themasking resist layer can be removed without damaging the underlyingmaterial of device layer 22. In one embodiment, the low energy laserablation can be performed using a laser system commonly used fortrimming electronic components and reworking electronic circuits. Thelow energy laser ablation can have a spot size of approximately 1 μm to10 μm, pulse energy of approximately 0.1 mJ to 2.0 mJ, and wavelengthsof approximately 266 nm to 1064 nm.

Furthermore, equipment for low energy laser ablation may be cheaper,more compact and more readily available than equipment for high energylaser ablation. This can allow the low energy laser ablation equipmentto be placed within a closer location to an area where steps S104 andS106 are performed which can lead to quicker processing and productionof vibrating mass 2 with the desired resonant frequencies. This canreduce the production time for the device incorporating vibrating mass 2and thus increase the number of devices incorporating vibrating mass 2that are produced within a given period of time. This can also eliminatethe need to remove the device from the clean production area, reducingthe likelihood of introducing particulate contamination.

In Step S116, vibrating mass 2 is etched to remove select amounts ofmass at select locations of vibrating mass 2 forming blind vias orcavities 16 as shown in FIG. 5. In one embodiment, vibrating mass 2 canbe etched using deep reactive ion etching. In another embodiment,vibrating mass 2 can be etched using deep reactive ion etching usingtime-sequenced etch and passivate chemistries such as the Bosch processusing sulfur hexafluoride, SF6, and octafluorocyclobutane, C4F8. Thesecavities 16 may be of any arbitrary shape, and may include, for example,cylinders, squares, or linear trenches. After etching vibrating mass 2,it is contemplated that the resonant frequencies of vibrating mass 2 canbe adjusted to approximately the desired resonant frequencies. Forexample, if vibrating mass 2 originally has a resonant frequency of14.950 kHz on the first axis and 14.900 kHz on the second axis, butvibrating mass 2 should have a common resonant frequency of 14.900 kHzon both axes, then after etching, vibrating mass 2 can have a resonantfrequency of approximately 14.900 kHz on both axes. In step S112, theprocess ends.

In some cases, vibrating mass 2 may have a frequency split even afterperforming the steps of FIG. 1. For example, vibrating mass 2 can have aresonant frequency of 14.9905 kHz on the first axis and 14.990 kHz onthe second axis when the desired common resonant frequency is 14.990kHz. The present invention, however, advantageously reduces thefrequency split of vibrating mass 2.

Furthermore by using the steps disclosed in FIG. 1, it is contemplatedthat vibrating mass 2 can remain in a production environment or foundrythroughout the entire process instead of being moved to a separatelocation. The minimization of movements of vibrating mass 2 can reducemanufacturing costs as moving vibrating mass 2 can be costly.Furthermore, the reduction of movement is beneficial in reducing thelikelihood that vibrating mass 2 will be damaged during the movingprocess by the introduction of particulates by virtue of its removalfrom a clean production environment.

It is also contemplated that the steps disclosed in FIG. 1 may beadvantageously used in devices where the device frequencies are testableafter Step S106 with masking layer 6 still in place and where the devicecan accommodate the over etch needed to remove the mass removal amountat the mass removal locations.

Although not depicted, in another embodiment, should the resonantfrequencies of vibrating mass 2 still be unacceptable, any or all ofSteps S106 to Step S112 can be repeated. That is, if the resonantfrequencies of vibrating mass 2 is 14.9905 kHz in the first axis and14.990 kHz in the second axis then vibrating mass 2 can have more massremoved in select new additional mass removal locations with a newadditional mass removal amount such that vibrating mass 2 has a resonantfrequency of 14.990 kHz in both axes. However, if the new resonantfrequencies are still unacceptable, then again, any or all of Steps S106to Step S112 can be repeated until vibrating mass 2 has suitableresonant frequencies. However, in subsequent applications of theprocess, the etching mask remains open in the original mass removallocations as well as any new mass removal locations defined. Thus,during subsequent etch processing mass will continue to be removed atthe original mass removal locations as well as at the newly defined massremoval locations. This should be taken into consideration in definingthe new mass removal locations and mass removal amounts.

FIG. 7 is a flow chart of an alternate embodiment of the presentinvention. In Step S702, the process to adjust the resonant frequenciesof a vibrating mass begins. The process depicted in FIG. 7 begins byutilizing vibrating mass 2 which is already etched at device layer 22 toform trenches 10 and which already has mask 6 removed as shown in FIG.8.

In Step S706, vibrating mass 2 is analyzed to determine if the resonantfrequencies of vibrating mass 2 need to be adjusted and locations ofdevice layer 22 where mass should be removed to achieve the desiredresonant frequencies of vibrating mass 2. For example, if vibrating mass2 has a resonant frequency of 14.950 kHz in a first axis and a resonantfrequency of 14.900 kHz in a second axis, then vibrating mass 2 shouldbe adjusted to decrease the resonant frequency of the first axis by 50Hz to bring the values for both the first axis and the second axis intoconformity at a common resonant frequency of 14.900 kHz.

To adjust the resonant frequencies of vibrating mass 2 select amounts ofmass from select locations of vibrating mass 2 can be removed. In StepS706, a mass removal amount of device layer 22 is determined while inStep S708, a mass removal location of device layer 22 is determined.

In Step S710, vibrating mass 2 is coated with a conformal masking orinsulation material 18 as shown in FIG. 9. Masking material 18 can be,for example, parylene. Parylene provides a vapor-deposited conformalcoating that can be deposited at low process temperatures. The drycoating process avoids the issues of capillary adhesion forces andparticulate contamination that are commonly associated with liquid-basedprocessing. The conformal coating ensures protective masking of theareas that are desired to remain non-etched. Parylene can also beremoved by dry processing using oxygen plasma, which facilitates removalafter the process is completed. Other materials that provide completecoating of the device top surface to provide the masking under etch cansimilarly be used. For example, in another embodiment the maskingmaterial may be a metal film deposited by sputtering, evaporation, oratomic layer deposition (“ALD”). It is preferred that this material canbe selectively removed upon completion of the process without damagingor degrading other parts of the device.

In Step S712, select locations of masking material 18 are removedcorresponding to the select locations of vibrating mass 2 as shown inFIG. 10. As shown in FIG. 10, select holes 20 are created in maskingmaterial 18 corresponding to select locations where mass from devicelayer 22 should be removed so that vibrating mass 2 can have the desiredresonant frequencies. In one embodiment, masking or insulation material18 is removed through laser ablation. Again the use of low energy laserablation can be beneficial compared to the use of high energy laserablation to reduce debris, reduce the likelihood of damage to underlyingmaterial 2, and reduce associated financial costs.

In Step S714, vibrating mass 2 is etched to remove select amounts ofmass at select locations of vibrating mass 2 forming cavities 16 asshown in FIG. 11. In one embodiment, vibrating mass 2 can be etchedusing deep reactive ion etching. In another embodiment, vibrating mass 2can be etched using deep reactive ion etching using time-sequenced etchand passivate chemistries such as the Bosch process. After etchingvibrating mass 2, it is contemplated that the resonant frequencies ofvibrating mass 2 can be adjusted to approximately the desired resonantfrequencies. For example, if vibrating mass 2 originally has a resonantfrequency of 14.950 kHz on the first axis and 14.900 kHz on the secondaxis, but vibrating mass 2 should have a common resonant frequency of14.900 kHz on both axes, then after etching, vibrating mass 2 could haveresonant frequencies of approximately 14.900 kHz on both axes.

In Step S716, the remaining masking material 18 in vibrating mass 2 isremoved as shown in FIG. 12. Masking material 18 can be removed, forexample, through etching. In the preferred embodiment in which themasking material is parylene, oxygen plasma etching may be used. In stepS718, the process ends.

It is contemplated that any or all Steps S704 through S716 can berepeated as necessary in order to further adjust the resonantfrequencies of vibrating mass 2.

It is also contemplated that the steps disclosed in FIG. 7 may beadvantageously used in devices where the original masking layer must beremoved prior to testing or where device over etch cannot beaccommodated.

In one embodiment, the process described in FIG. 1 and FIG. 7 can becombined. That is, after the process in FIG. 1 is completed and theresonant frequencies of vibrating mass 2 still needs to be adjusted,etch mask 6 can be removed after Step S116 in FIG. 1 and instead ofending the process, the process can go to Step S704 with the addition ofa layer of masking material 6. Thus, the initial frequency tuning couldbe accomplished using the process disclosed in FIG. 1, and subsequentfine adjustments done with the process disclosed in FIG. 7.

Advantageously the processes described in FIG. 1 and FIG. 7 permit thefrequency tuning to be accomplished at the wafer scale. This allows thewafer of tuned devices to continue wafer-scale processing, preservingthe cost benefits inherent to batch processing. In particular,wafer-scale vacuum packaging is an attractive process for sealingresonator devices in an environment that allows operation with highquality factor. The processes described in FIG. 1 and FIG. 7 maintainthis compatibility.

What is claimed is:
 1. A method for adjusting the resonant frequenciesof a vibrating MEMS device comprising the steps of: patterning a surfaceof a structural layer used to form the vibrating MEMS device with aphotoresist, the photoresist having open areas where the structurallayer used to form the vibrating MEMS device should be etched; etchingthe structural layer used to form the vibrating MEMS device at locationscorresponding to the open areas to structurally define the vibratingMEMS device; determining a first resonant frequency of the vibratingMEMS device along a first axis and a second resonant frequency of thevibrating MEMS device along a second axis; determining a first massremoval amount of the vibrating MEMS device and a first mass removallocation of the vibrating MEMS device to alter the first resonantfrequency and reduce a resonant frequency difference between the firstresonant frequency and the second resonant frequency; removing thephotoresist at the first mass removal location using laser ablation;etching the vibrating MEMS device to remove the first mass removalamount of the vibrating MEMS device at the first mass removal locationof the vibrating MEMS device; determining a third resonant frequency ofthe vibrating MEMS device along the first axis and a fourth resonantfrequency of the vibrating MEMS device along the second axis;determining a second mass removal amount of the vibrating MEMS deviceand a second mass removal location of the vibrating MEMS device to alterthe third resonant frequency and reduce a resonant frequency differencebetween the third resonant frequency and the fourth resonant frequency;coating a portion of the surface of the vibrating MEMS device with amasking material; removing the masking material at the second massremoval location using laser ablation; and etching the vibrating MEMSdevice to remove the second mass removal amount of the vibrating MEMSdevice at the second mass removal location of the vibrating MEMS device.2. The method of claim 1 wherein the vibrating MEMS device is a MEMSresonator.
 3. The method of claim 2 wherein the MEMS resonator is a MEMSgyroscope resonator.
 4. The method of claim 1 further comprising thestep of removing any remaining photoresist that is patterning thesurface of the vibrating MEMS device after the etching the vibratingMEMS device at the first mass removal location of the vibrating MEMSdevice.
 5. The method of claim 1 further comprising the step of removingany remaining masking material that is coating the surface of thevibrating MEMS device after the etching the vibrating MEMS device at thesecond mass removal location of the vibrating MEMS device.
 6. The methodof claim 5 wherein the removing any remaining masking material that iscoating the surface of the vibrating MEMS device is performed by oxygenplasma treatment.
 7. The method of claim 1 wherein the steps ofadjusting the resonant frequencies of the vibrating MEMS device isperformed at a wafer scale.
 8. The method of claim 1 wherein the maskingmaterial is a conformal masking material.
 9. The method of claim 8wherein the conformal masking material is selected from the family ofparylenes (poly-para-xylenes).
 10. The method of claim 1 wherein theetching the vibrating MEMS device at the first mass removal location ofthe vibrating MEMS device is performed using an etch process thatminimizes material removal at locations other than the first massremoval location.
 11. The method of claim 1 wherein the removing thephotoresist at the first mass removal location using laser ablation isperformed with an energy sufficient to remove the photoresist withoutdamaging the structural layer used to form the vibrating MEMS device.12. The method of claim 1 wherein the etching the vibrating MEMS deviceat the second mass removal location of the vibrating MEMS device isperformed using an etch process that minimizes material removal atlocations other than the second mass removal location.
 13. The method ofclaim 1 wherein the etching the vibrating MEMS device at the first massremoval location or etching the vibrating MEMS device at the second massremoval location is performed using deep reactive ion etching.
 14. Themethod of claim 1 wherein the determining the first mass removal amountof the vibrating MEMS device and the first mass removal location of thevibrating MEMS device is performed using the masking material.
 15. Themethod of claim 1 wherein the etching the structural layer used to formthe vibrating MEMS device is performed using deep reactive ion etching.